Novel composite based on silicone rubber and a nano mixture of SnO2, Bi2O3, and CdO for gamma radiation protection

Recently, there has been a surge of interest in the application of radiation-shielding materials. One promising research avenue involves using free-lead metal oxides/polymer composites, which have been studied for their radiation shielding and characterization properties. This study reinforced the dimethylpolysiloxane (silicone rubber) composites with micro- and nano-sized particles of tin oxide, cadmium oxide, and bismuth oxide as additive materials. The composites were tested with 20 and 50 weight fractions, and their attenuation coefficients were measured using a NaI(TI) detector at gamma-ray energies ranging from 59.54 to 1408.01 keV. Also, the thermal and mechanical properties of the composites were observed and compared with those of free silicone rubber. The results showed that the 50% nano metal oxide/SR composites exhibited better thermal stability and attenuation properties than the other composites, also possessing unique attributes such as lightweight composition and exceptional flexibility. Consequently, this composite material holds immense potential for safeguarding vital organs, including the eyes and gonads, during radiological diagnosis or treatment procedures. Its exceptional ability to absorb a significant portion of incident rays makes it an invaluable asset in the field of radiation protection.


Materials
The silicone rubber was utilized as a polymer.By adding stiffener with a 2% weight fraction, silicone rubber transformed into a rigid composite by catalyzed reaction.The additive metal oxides (Bi 2 O 3 , CdO, and SnO 2 ) have the same fraction in the mixture.The nano metal oxides were supplied by Nanotech Company, Egypt.Bi, Sn, and Cd are heavy metals characterized by their high density and high atomic number.However, the utilization of cadmium oxide (CdO) raises significant concerns due to its inherent toxicity and potential adverse environmental impact.But in radiation therapy using high-energy photon beams (E > 10 MeV), neutrons are primarily generated in the linac head through interactions known as (γ,n), where photons interact with the nuclei of high atomic number materials present in the linac head and the beam collimation system.The presence of these neutrons impacts the necessary shielding requirements within radiation therapy rooms and also leads to an increase in the out-of-field radiation dose for patients undergoing radiation therapy with high-energy photon beams.The presence of Cd will help absorb these neutrons, especially the thermal ones.
Silicone rubber is produced commercially.Enhanced production of silicone rubber involves the incorporation of nanomaterials, which unfortunately leads to an increase in costs.However, a promising solution to mitigate these expenses lies in the utilization of the ball milling method.This technique enables the transformation of bulk materials into nanoscale counterparts, thereby reducing overall production costs.
To protect the environment, it shouldn't dispose of silicone items indiscriminately.Instead, make a better choice by sending silicone items to specialized recycling companies.Silicone rubber is a durable material and recycled many times.Also, it can send them off to your local recycling centers to get them properly recycled [17][18][19] .

Synthesis of metal oxides/SR composites
The metal oxides (Bi 2 O 3 , CdO, and SnO 2 )/SR samples were prepared by mixing and molding.The silicone rubber is loaded with micro-and nano-(Bi 2 O 3 , CdO, and SnO 2 ) by mixing for different weight fractions.After that, add a stiffener "vulcanizing agent" to the mixture at a concentration of 2%.To remove air bubbles from the matrix, the matrix was vacuumed for 30 min.The samples were placed in laboratory to 48 h to dry then employed in experimental steps, then mold at room temperature for 24 h to obtain metal oxides/SR samples 20,21 .

Scanning and transmission electron microscope
The size of the nano fillers was analyzed using a transmission electron microscope (FE-TEM) manufactured by JEOL, Japan, operating at 200 kV.The size of micro fillers was measured using a scanning electron microscope (SEM) [JSM-6010LV, JEOL].Furthermore, the scanning electron microscope (SEM) was employed to observe the distribution of micro-and nano-sized particles (Bi 2 O 3 , CdO, and SnO 2 ) within the composites' cross-section [22][23][24] .

Thermogravimetric analysis (TGA)
The thermal properties of composites were investigated by a TGA [SDT-Q600] machine.Changes in thermal stability of micro-and nano-(Bi 2 O 3 , CdO, and SnO 2 )/SR were tested at 10 °C/min heating rate as a function of temperature from 25 to 800 o C [28][29][30] .

Scanning and transmission electron microscope
Figure 2 showcases an image captured by a scanning electron microscope (SEM), displaying micro-sized particles of cadmium oxide rods, tin oxide, and bismuth oxide particles.Furthermore, it presents a transmission electron microscope (TEM) image, revealing nano-sized of these particles.The average size of micro metal oxides is in the range of 1.038-6.186μm as shown in Fig. 2a, c, and e while Fig. 2b, d, and f show the average size of nanometal oxides in the range of 8.67-27.93nm. Figure 3 represents the SEM micrographs of SR, micro-and nanocomposites.From Fig. 3a, the free silicone rubber cross-section has a clear and regular structure, while the reinforced silicone rubber composites have a more erratic and rough structure.but at nanocomposites, nano-(Bi 2 O 3 , CdO, and SnO 2 ) were better dispersed than micro-(Bi 2 O 3 , CdO, and SnO 2 ) in the silicone rubber matrix.When the additive material weight fraction increases from 20 to 50%, agglomerations of filler increase in micro-composites and interparticle distances become smaller, which affects mechanical properties.

Mechanical results
Figure 4 shows stress-strain curves of free silicone rubber and metal oxides/SR composites.The additives (Bi 2 O 3 , CdO, and SnO 2 ) improve the ultimate stress and tensile strength of silicone rubber composites.Moreover, with an increment in filler weight fraction, first the ultimate stress and tensile strength increase at 20% concentration   and then decrease at 50% concentration because of the agglomeration of filler particles in silicone rubber, which decreases the strain of SR composites.As described in Fig. 4, tensile strength of SR composites follows the order: free-SR < 50% nano < 50% micro < 20% nano < 20% micro, where the ultimate stress of micro-SR composites is higher than that of nano-SR composites at the same concentration.At 50% micro and nano concentrations, ultimate stress and strain tend to decrease at 50%, so that the expected enhancement does not occur and tensile properties will decrease.Therefore, the limitation line for silicone rubber must be achieved at less than 50% concentration.

Thermogravimetric analysis results
It's known that metal oxides have high-temperature stability.So when (Bi 2 O 3 , CdO, and SnO 2 ) are added to silicone rubber composites.It has ameliorated and improved the thermal stability of composites.Figure 5 represents the TGA and differential TGA results of (Bi 2 O 3 , CdO, and SnO 2 )/SR composited.For free silicone rubber, it has thermal stability at around 300 o C but after 450 o C it gradually decomposes, and weight loss reaches 71.68%.By adding metal oxides, the thermal stability of silicone rubber composites is overall improved, and the weight loss percentage decreases.Moreover, thermal stability increases with increasing weight fractions of metal oxides at the micro and nanoscales.Figure 5a shows nanocomposites have lower weight loss and better thermal properties than micro composites at the same weight fraction.Figure 5b shows that the DTGA peak decreases as an additive material increases because of the presence of inorganic composites in a mixture where the specific heat capacity of metal oxides is much larger, and the heat absorption efficiency is higher.

Shielding results
The measured result of MAC of metal oxides (Bi 2 O 3 , CdO, and SnO 2 )/SR and theoretical data utilizing XCOM software are listed in Table 2.The deviation values were estimated for each concentration to explain the acceptable harmony percentage between two values for all energies, where deviation for silicone rubber is in the range of − 3.633 to 4.302, for 20% (Bi 2 O 3 , CdO, and SnO 2 )/SR between − 3.921 and 2.493, and for 50% (Bi 2 O 3 , CdO, and SnO 2 )/SR between − 3.099 and 2.122.This harmony indicates the validation of the experimental system.Figure 10 describes the Z eff for silicone rubber, 20% Bi 2 O 3 , CdO, and SnO 2 /SR, and 50% Bi 2 O 3 , CdO, and SnO 2 /SR.Z eff lines describe the attenuation ability of composites, which depends on the energy of the beam and on the Z of the elements in composites.It shows that the 50% Bi 2 O 3 , CdO, and SnO 2 /SR composite has the highest Z eff , which is reinforced with 50% Bi 2 O 3 , CdO, and SnO 2, while the silicone rubber composite has the lowest Z eff .The probability of interaction between beam photons and material depends on Z, where the photoelectric effect is directly proportional to Z 4 , Compton scattering depends on Z, and pair production interaction is influenced by Z 2 .So as the weight fraction of the high Z filler "Bi 2 O 3 , CdO, and SnO 2 " increases, the Z eff increases, and attenuation increases.
Table 3 illustrates a comparison between recently published data and our current research on the improvement of attenuation properties using nanocomposites for gamma ray applications.According to the findings presented in Table 3, the utilization of nanoparticles significantly improves the attenuation properties.In the current study, the incorporation of nano-Bi 2 O 3 , CdO, and SnO 2 /SR composition results in an impressive attenuation of 33.58%  for low energy and 16.47% for high energy.This remarkable enhancement can be attributed to the high density of the composite (2.122 g/cm 3 ).

Conclusion
This study aims to investigate the impact of particle size and weight fraction of micro and nano (Bi 2 O 3 , CdO, and SnO 2 ) on the linear and mass attenuation coefficient of the metal oxide/SR composite at various photon energies.By measuring these coefficients, we can gain valuable insights into the behavior of the composite material.By employing advanced techniques such as SEM and TEM to the morphology of the composites  and filler materials, the study reveals that nano composites exhibit a more uniform morphology compared to micro composites.This finding suggests that nano composites possess superior effectiveness in shielding against radiation.Furthermore, the study highlights the significance of tailoring composites with appropriate properties for efficient shielding by demonstrating that the size and concentration of filler materials impact the density of the composites.As the density of composites increases, their ability to attenuate radiation also increases.This observation underscores the importance of creating composites with optimal density to maximize their shielding capabilities.Moreover, this study delves into the impact of changes in the weight fraction of fillers on the tensile parameters of the composites, providing valuable insights into the mechanical properties of these materials.By understanding how variations in the weight fraction of fillers affect the tensile parameters, researchers and engineers can make informed decisions when designing and utilizing these composites.

gFigure 2 .
Figure 2. (a, c, and e) SEM image of micro-tin oxide particles, micro-cadmium oxide, and micro-bismuth oxide respectively.(b, d, and f) TEM image of nano-tin oxide particle, nano-cadmium oxide rods, and nanobismuth oxide respectively.

Figure 6 .
Figure 6.Comparison between LAC for nano-and micro-Bi 2 O 3 , CdO, and SnO 2 /SR at different energy photons at 20% and 50% weight fraction as a function of density.

Figure 7 .
Figure 7. HVL of silicone rubber, micro-and nano-Bi 2 O 3 , CdO, and SnO 2 /SR for different weight fraction at different energy.

Figure 8 .
Figure 8. TVL of silicone rubber, micro-and nano-Bi 2 O 3 , CdO, and SnO 2 /SR for different weight fraction at different energy.

Figure 9 .
Figure 9. MFP of silicon rubber, micro-and nano-Bi 2 O 3 , CdO, and SnO 2 /SR for different weight fraction at different energy.

Figure 10 .
Figure 10.The effective atomic number of silicone rubber, micro-and nano-Bi 2 O 3 , CdO, and SnO 2 /SR for different weight fractions at different energy. .

Table 1 .
Evaluation equation of radiation shielding parameters for silicone rubber composites.